FIG. 1 is a schematic illustration of a part of a cellular communication network in communication with a User Equipment (UE) 120. The cellular communication network comprises a base station (e.g. NodeB) 110 and a radio network controller (RNC) 100. The base station and the RNC may or may not be co-located and may be implemented within a same device or in different devices. A radio link 121 transports information over the air between the base station 110 and the user equipment 120 and a transport link or transport network (TN) 101 connects the base station 110 and the RNC 100 and is used to transport information there between.
Traffic congestion may appear in a cellular communication network. For example, in a network applying High-Speed Downlink Packet Access (HSDPA) capacity limiting congestion may arise on the radio link and/or in the transport network.
The Internet protocol Transmission Control Protocol (TCP) provides an efficient congestion control mechanism. However, the TCP cannot be used directly to resolve congestion situations between the RNC and the UE when HSDPA is applied. This is due to that the Radio Link Control (RLC) protocol will be in Acknowledged Mode (AM) when HSDPA is applied. Hence, since RLC AM provides re-transmission of packets that are lost in transmission between the RNC and the UE due to a congestion situation, TCP will not register any packet losses and will not initiate its congestion avoidance mechanism. Contrarily, TPC will typically continue to feed more packets to the lower layers and thereby contribute even more to the congestion.
Even though the problem and suggested embodiments herein will be exemplified in the context of HSDPA and TPC, the same situation may appear in connection with other known or future protocols based on retransmission being applied together with other known or future protocols with congestion avoidance based on detection of corrupted or lost packets.
In HSDPA, the RLC AM protocol is responsible for reliable data transmissions in Layer 2 between the RNC and the UE (see e.g. Third Generation Partnership Project (3GPP) Technical Specification (TS) 25.322). An AM RLC entity can act either as a sender or as a receiver depending on the applicable elementary procedure, where the sender transmits RLC AM Protocol Data Units (PDUs) and the receiver receives and processes the RLC AM PDUs. The RLC AM provides reliable transmission and seamless handover and channel switching operations. Data packets lost between the RNC and the UE (e.g. due to bad link conditions of the radio link and/or the transport network link) are re-transmitted. Thus, the RLC AM provides avoidance (or at least minimization) of packet losses, which is beneficial for TCP in terms of utilizing network capabilities, at the cost of a possible increase in average round trip time (RTT). However, if packets are lost due to congestion, the re-transmission protocol will typically keep re-transmitting packets which will add further to the congestion situation and conceal the congestion situation to the TCP.
Some solutions exist to handle congestion in the transport network of a HSDPA system. For example, the Iub Framing Protocol (FP) may be used to detect congestion at the base station and standardized signaling messages may be used to inform the RNC about the detected congestion.
A detailed description of TN congestion detection and control mechanisms and the Iub Framing Protocol may be found, for example, in “HSDPA/HSUPA Handbook”, CRC Press 2010, edited by Borko Furht and Syed A. Ahsson, pp. 297-330, Chapter 9, “HSPA Transport Network Layer Congestion Control”, by Szilveszter Nádas, Sándor Rácz and Pál L. Pályi.
Since a lost (or dropped) packet is re-transmitted by the RLC AM protocol, a lost RLC PDU does not result in a RLC Service Data Unit (SDU) loss, which would be beneficial to inform the TCP about a congestion situation.
In a first example congestion avoidance approach (see e.g. WO2011/120581), the RNC may drop an application level packet (e.g. RLC SDU) when it is informed about the detected congestion to notify the TCP about the congestion situation.
In a second example congestion avoidance approach, the NodeB does not necessarily inform the RNC about a detected congestion, but deliberately corrupts a RLC PDU (e.g. the next PDU) before transmission to the UE over the radio link. This will result in an erroneous TCP Cyclic Redundancy Check (CRC) at the UE and, thus, the TCP is notified about the congestion situation.
See also e.g. WO2012/115545 and WO2013/070162 for other example congestion avoidance approaches.
There is a need for alternative approaches to avoidance of transport network congestion when a re-transmission protocol is applied.